The present invention relates to water heaters and, more particularly, to those which provide hot water continuously without the need for a storage tank.
Water heaters are well known and generally include a storage tank, a thermostat, a heat source and inlet and outlet ports. The water in the tank is heated until it reaches a preset temperature controlled by the thermostat.
Because the water is heated in a relatively large tank, the heating rate of these conventional storage heaters is relatively low. Water is not heated at the same rate it is used. Instead, heat is applied to water in the tank so that a relatively long period of time is required to heat the water to the desired temperature.
The storage tank provides a reserve of heated water, which is used to supply short term needs. If more hot water is used than that in the tank, the outlet water temperature drops dramatically because of the low heating rate of the unit. This requires a close approximation of the amount of hot water that has to be used in one interval. When the water flow is stopped, the heater once again heats water in the storage tank to the desired temperature and therefore insures a sufficient hot water supply for the next use.
This arrangement requires the storage tank to be located in an environment with an ambient temperature lower than that of the water in the tank. The tank tends to lose heat to the ambient air, thus lowering the water temperature and requiring the heating element to reheat the water. This energy is lost to the environment and provides no tangible benefits.
One solution to this problem has been to better insulate the storage water heaters. This moderately reduced the amount of heat lost to the environment, but did not eliminate all heat loss and also took up additional space.
A second solution has been various configurations of tankless water heaters. These units did not have a storage tank, but heated the water as it flowed through the device. This arrangement eliminated most of the storage-tank heat losses. The space problem would also be solved because the need to store a large volume of water was removed. An unlimited supply of hot water was also now available, because it could continuously flow through the tankless system.
However, problems were present in these units. For a given energy input, temperature rise was proportional to water flow rate. Most units were small capacity units, having limited flow rates or temperature rises. The larger units had satisfactory maximum flow rates and maximum temperature rises, but also required larger minimum flow rates before they became operational. If they were turned on at lower flow rates, the water would overheat by the time the water reached the outlet port. This aspect limited their use to situations having relatively high minimum flow rates.